460 Gary N. Calkins. 



EXPLANATION OF PLATES. 



All of the photographs were taken by Dr. Edward Learning from permanent preparations of Para- 

 mcecium caudatum, stained with picro-carmine. All are equally magnified and the relative sizes 

 represent absolute differences. 



Plate I. 



Figs. I and 2. Two normal specimens B series (107th generation and after three months of cul- 

 ture in hay infusion. These do not differ from typical Paramoecium from the ponds, and have many 

 endoplasmic vacuoles, alveolar protoplasm, and homogeneous nuclei. 



Fig. 3. A typical individual of the B series during the first period of depression. The ectoplasm 

 is fully as clearly defined, and as thick as in the largest forms, indicating that this portion at least, has 

 not suffered from degeneration, a result differing from that in starved forms. (Compare Figs. 23 

 and 24). 



Fig. 4. An individual from the B series in the 306th generation, stimulated with beef extract in 

 August, fed continuously with hay infusion for three months until killed. The endoplasm is filled with 

 gaGtric vacuoles and with partly digested food, the dissociated or "labile" condition of the endoplasm 

 shown here is characteristic of Paramoecium under normal conditions. 



Fig. 5. An individual from the A series during the third cycle (550th generation), and twenty-four 

 hours after treatment with beef extract. The endoplasm is filled with gastric vacuoles, the macronu- 

 cleus is normal, but the micronucleus has divided three times and a clump of six nuclei may be seen at 

 the lower end. There is a tendency toward a denser structure of the endoplasm, especially at the 

 two extremities, this being indicative of approaching physiological depression. 



Fig. 6. An individual from the A series in the 560th generation. Treated 48 hours before fixation 

 with beef extract. Gastric vacuoles are abundant in the upper portion, but in the lower part the 

 characteristic density which marks the climax of physiological depression is shown, /. e., an apparently 

 general "loading" of the protoplasm with inert material. 



Fig. 7. An individual from the A series in the 615th generation killed at a time of general depres- 

 sion. It shows the typical condensed appearance when the power of division is lost and leads to death 

 after several days without division. 



Fig. 8. An individual from the A series in the 623d generation (June, 1902,) and 24 hours after 

 successful stimulation with extract of pancreas. The condition shown in Fig, 7 has been successfully 

 overcome, and activity renewed by this treatment. This and the two following figures show stages in 

 the breaking up of this dense, endoplasmic mass. The macronucleus is divided while the ends alone of 

 the animal still retain the densely granular character. 



Figs. 9 and 10. Two individuals from the A series 48 hours after successful stimulation with 

 pancreas extract. The endoplasm is now in a "labile" condition, although the extremities are still 

 dense. The individual shown in Fig. 10 is further advanced in recovery than that shown in Fig. 9, 

 but both are sister cells of individuals that carried the race to the 742d generation. 



Plate II. 



Fig. II. Two individuals of the A series in the 604th generation, two weeks prior to the fatal 

 depression of June, 1902. These were treated with a weak solution of dibasic potassic phosphate (see 

 paper) for 30 minutes and then transferred to hay infusion and killed 24 hours afterwards. 



Fig. 12. Two individuals of the A series treated at the same time as the preceding, with a weak 

 solution of magnesium chloride for 25 minutes. The protoplasmic structures are normal and the endo- 

 plasm has the typical alveolar appearance. 



